Recently, in a field of the ultrasonic flaw detection method for inspecting various structural materials, there are developed flaw detection techniques capable of visualizing an internal structure of an object to be inspected, in a short time with high precision, thereby to inspect the internal structure thereof, as typified by a phased array technique, an aperture synthesis and the like (e.g., see Non-patent document 1: Norimasa Kondou, Yoshimasa Oohashi, and Akio Jitsumori; “Digital signal processing in measurement and sensors” of Vol. 12 of Digital Signal Processing Series; SHOKODO CO., LTD., 1993, pp. 143-186).
The phased array technique uses a so-called ultrasonic array probe comprising an array of piezoelectric elements (piezoelectric transducers). The phased array technique is based on the following principle: respective piezoelectric elements issue ultrasonic waves, thereby, wavefronts of these ultrasonic waves interfere with each other to form a synthetic wavefront, and then synthetice wavefront travel with propagation. Therefore, provided that ultrasonic issue timings of the respective piezoelectric elements are shifted from each other by making delay control for each ultrasonic issue timing, it possible to control an incidence angle of each ultrasonic wave and focus the respective ultrasonic waves.
Even on a receiving side of the ultrasonic wave, provided that reflected ultrasonic waves received by the respective piezoelectric elements are shifted from each other and then added, similarly to the issue side of the ultrasonic wave, it is possible to control each received incidence angle of the ultrasonic wave and receive a focused ultrasonic wave.
The phased array technique is generally known as a linear scan technique and a sector scan technique. The linear scan technique linearly scans piezoelectric elements of a one-dimensional array probe. The sector scan technique varies issue and reception directions of the ultrasonic wave in sector -like fashion. When using a two-dimensional array probe comprising piezoelectric elements arranged in a lattice-like formation, it makes possible to focus on any position three-dimensionally and provide a scan technique capable of suiting to any object to be inspected. Any of these techniques can realize fast scan of ultrasonic waves without moving an ultrasonic probe or control any incidence angles or focus depths of the ultrasonic wave without replacing the ultrasonic probe.
The phased array technique is capable of fast and high-precision inspections.
The aperture synthesis is based on the following principle. When issuing an ultrasonic wave so as to widely diffuses within an object and receiving its reflected ultrasonic wave signal, a defect position corresponds to a sound source of the received reflected ultrasonic wave, and is located along an arc whose center is a piezoelectric element used to issue and receive the ultrasonic wave and which has a radius equivalent to the propagation distance of the reflected ultrasonic wave. Therefore, provided that the ultrasonic wave is issued and received while sequentially changing a position of the piezoelectric element, and a received waveform at each position of the piezoelectric element is spread in an arc by calculation of a computer, intersection points of the arcs concentrate on the defect position as a source of reflecting the ultrasonic wave, thereby making it possible to identify the defect position. Contents of the calculation processes of the computer are explained in Non-patent Document 1.
These methods using the probe comprised of multiple piezoelectric elements and are capable of three-dimensionally receiving a reflected ultrasonic wave signal for the defect without moving the probe. The reflected ultrasonic wave signal is used to identify a three-dimensional reflection position. The reflection position may be estimated by displaying two-dimensional images representing multiple reflection intensity distributions at different spatial positions or converting the reflection intensity into three-dimensional data and then stereoscopically displaying an image. For example, the linear scan technique or the sector scan technique as the phased array technique can generate multiple two-dimensional reflection intensity images corresponding to known scanning pitches. Images can be sequentially changed on a screen to identify a direction along which the reflected wave appears. However, the phased array technique indicates limitations on any three-dimensional scans other than the above.
In such case, a known method interpolates reflected ultrasonic wave signals from multiple directions to make three-dimensional lattice data, and displays an image for the data based on volume rendering or surface rendering. A three-dimensional lattice data structure called as voxel, which multiple cubes are three-dimensionally arranged, is most widely used because it can be easily processed. The voxel is also referred to as a structured lattice. An alternative to the voxel is a lattice that is irregularly positioned as a spatial array and is less easily displayed than the voxel. Such lattice is also referred to as an unstructured lattice. Typical unstructured lattices include a hexahedron lattice, a tetrahedron lattice, a triangular prism lattice, and a four-sided pyramid lattice. Another method displays reflected ultrasonic wave signals as a group of three-dimensional dots without conversion into lattice data. These types of data are maintained as three-dimensional flaw detection data. An observer can confirm the three-dimensional flaw detection data from any direction after measurement (e.g., see Non-patent Document 2: Potts, A.; McNab, A.; Reilly, D.; Toft, M., “Presentation and analysis enhancements of the NDT Workbench a software package for ultrasonic NDT data”, REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION: Volume 19. AIP Conference Proceedings, Volume 509, pp. 741-748 (2000)) .
However, it is difficult to determine whether the reflection intensity distribution peak results from reflection on an end face or a boundary surface of an object or from reflection on a defect only based on the three-dimensional flaw detection data. Another technology which concurrently displays three-dimensional shape data with three-dimensional flaw detection data for the object has been developed, and which superimpose and compares the two types of data. The technology facilitates distinction between a reflected ultrasonic wave signal (shape echo) dependent on a shape and a reflected ultrasonic wave signal (defect echo) from a defect. In many cases, the technology uses three-dimensional shape data that is generated and read with an additional general-purpose CAD (Computer Aided Design) system (e.g., see Non-patent Document 2).
Non-patent Document 1: Norimasa Kondou, Yoshimasa Oohashi, and Akio Jitsumori.; “Digital signal processing in measurement and sensors”. Vol. 12 of Digital Signal Processing Series; SHOKODO CO., LTD., 1993, pp. 143-186.
Non-patent Document 2: Potts, A.; McNab, A.; Reilly, D.; Toft, M., “Presentation and analysis enhancements of the NDT Workbench a software package for ultrasonic NDT data”, REVIEW OF PROGRESS IN QUANTITATIVE NONDESTRUCTIVE EVALUATION: Volume 19. AIP Conference Proceedings, Volume 509, pp. 741-748 (2000)
When the object is shaped complexly, however, the ultrasonic wave multiply reflects inside the object, causing many shape echoes to appear. In such a case, it is difficult to distinguish between a shape echo and a defect echo even though a flaw detection result is superimposed on the shape data. In an actual flaw detection process, the defect determination evaluation requires only a defect echo and a restricted shape echo for identifying positional relation with the three-dimensional shape data. Depending on viewing directions for the three-dimensional display, the echoes needed for the evaluation may overlap with the other unnecessary echoes (false echoes) to be displayed. A false echo may hinder the evaluation.
The present invention is to provide an ultrasonic flaw detector and an ultrasonic flaw detection method capable of eliminating a false echo from a three-dimensional display as a flaw detection result and displaying only a defect echo and a shape echo needed for evaluation.